1. Field of the Invention
The present invention relates to an optical disc units for reproducing data recorded on any of different density recording media such as a high density optical disc or a compact disc, or recording data on the recording medium, and also related to an optical pick-up used in an optical disc unit and using a liquid crystal element.
2. Description of the Related Art
First, explanation will be made of a liquid crystal display element used in a conventional optical pick-up in a DVD unit. Referring to FIG. 8 which is a plan view illustrating an electrode pattern for liquid crystal in a conventional liquid crystal display element, the electrode pattern is composed of three divided zones, that is, a reference zone {circle around (2)}, and zones {circle around (1)} and {circle around (3)} which are applied thereto respectively with voltages that is equal to each other, but having opposite polarities so as to cause a phase difference in transmitted light beams passing through the zones {circle around (1)} and {circle around (3)} with respect to the reference zone {circle around (2)}.
Next, referring to FIG. 9 which is a perspective view illustrating an optical pick-up on which the liquid crystal element shown in FIG. 8 is mounted, the optical pick-up is composed of a semiconductor laser 15, a beam splitter (which will be herein below simply denoted as xe2x80x9cBSxe2x80x9d) 16, a collimator lens 17, a rise-up miller 18, the liquid crystal element 19, an objective lens 20, a detection lens 21, and a detector 22. Further, the semiconductor lens 15 emits a laser beam having a wavelength of 650 nm, which is then transmitted through the BS 16 and then through the collimator lens 16 so as to be turned into a parallel ray beam. The parallel ray beam is totally reflected by the surface of the mirror 18, and is then transmitted through the liquid crystal element 19. As mentioned above, at this stage, phase differences are caused by beams transmitted through the zone {circle around (1)} and the zone {circle around (3)} with respect to a beam transmitted through the zone {circle around (2)}. Further, these beams are focused by the objective lens 20 into a photo spot on an optical disc.
Next, a reflected beam from the optical disc is incident upon the BS 16 after passing along the path reverse to the above-mentioned path. At this stage, by passing through the liquid crystal element 19, the above-mentioned phase differences are again caused. Thereafter, the reflected beam again reflected by the BS 16 is focused by the detecting lens 21 and is then received by the detector 22.
Thus, with the DVD pick-up in which the liquid crystal display element 19 having the electrode pattern composed of the three zones is located on the optical path between the collimator lens 17 and the objective lens 21, an optical disc can be subjected to tilt correction.
It is noted here that the tilt doses means such a condition that the optical beam focused by the objective lens 20 is incident upon a data recording surface (more precisely, a recoding layer) of the optical disc at an angle other than 90 deg. (that is, the optical axis is inclined). Further, the correction of an optical system from the tilt condition into a condition in which the optical axis is incident upon the surface of the optical disc, ideally perpendicular thereto by carrying out any control is so-called as the tilt correction.
However, the liquid crystal element having the above-mentioned conventional electrode pattern as mentioned above, has been adapted to be used for DVD discs. However, there has been presented such a problem that, an optical pick-up for recording and reproduction of optical discs having different recording densities, can carry out the above-mentioned tilt correction can not for all of them but for only one of them.
An object of the present invention is to provide an optical pick-up which can carry out tilt correction for each of optical discs having different recording densities, and which can enlarge the tilt margin for the optical discs, and reduce the accuracy for assembly and adjustment of the pick-up, and manhours for the assembly and the adjustment, and to provide an optical disc unit using such an optical pick-up.
The present invention is devised in order to solve the above-mentioned problems, and accordingly, there is provided an optical pick-up composed of an objective lens and a liquid crystal element in combination, for recording and reproduction of optical discs having different recording densities, characterized in that the liquid crystal element has an electrode pattern divided into five zones including second and third zones which are located respectively in center parts of half circles of a small diameter beam for a low density optical disc, fourth and fifth zones which are located substantially in the half circles of the small diameter beam along the outer peripheral parts of the second and third zones so as to surround the second and the third zones, respectively, and a first zone which are the remaining zone other than the second to fifth zones, including the diameter, the direction of the diameter included in the first zone is the same as a tilt direction to be corrected.